International Journal of Engineering Trends and Technology (IJETT) – Volume 11 Number 9 - May 2014
Cascaded MLI Operation for SPWM and Third
Harmonic SPWM
E.SaiPrateekReddy1 MRamGopal Reddy2 KRamBabu3 BNishanth4 ChPunya VenkataVarun5
12345
Dept. of EEE, KLUniversity, Vaddeswaram, Guntur, A.P. India
Abstract:
The multilevel inverters have gained
more interest these days because of their advantages
like voltage stress on each switching device is reduced,
easy to reach high voltage levels in high power
applications with lower harmonic distortion and
switching frequency, which is very difficult to get this
performance with conventional two level inverters. In
this paper three level cascaded multilevel inverter is
simulated and switching strategies like sinusoidal
pulse width modulation and third harmonic pwm are
implemented to it. The variation of output voltage
and harmonic distortion is observed. The results are
interpreted using SMULINK.
Keywords: low harmonics, voltage stress, cascaded,
spwm, third harmonic spwm.
I.
INTRODUCTION
According to four-switch combination, three output
voltage levels, +V, -V, and 0,can be synthesized for
the voltage across A and B. During inverter
operation shown in Fig.1, switch of S1 and S4 are
closed at the same time to provide VAB a positive
value and a current path for Io. Switch S2 and S4
are turned on to provide VAB a negative value with
a path for Io. Depending on the load current angle,
the current may flow through the main switch or
the freewheeling diodes. When all switches are
turned off, the current will flow through the
freewheeling diodes. In case of zero level, there are
two possible switching patterns to synthesize zero
level, for example, 1) S1 and S2 on, S3 and S4 off,
and 2) S1 and S2 off and S3 and S4 on.
The smallest number of voltage levels for a
multilevel inverter using cascaded inverter with
SDCSs is three. To achieve a three-level waveform,
a single full-bridge inverter is employed. Basically,
a full bridge inverter is known as an H-bridge cell,
which is illustrated in Fig.1[3]
Fig.1 Single H-Bridge Cell
ISSN: 2231-5381
Fig.2 Output of single H-Bridge and corresponding
switching pattern
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International Journal of Engineering Trends and Technology (IJETT) – Volume 11 Number 9 - May 2014
Fig.3 Three phase three level cascaded configuration
II.
SINUSOIDAL PWM
The sinusoidal pulse-width modulation (SPWM)
technique produces a sinusoidal waveform by
filtering an output pulse waveform with varying
width. A high switching frequency leads to a better
filtered sinusoidal output waveform. The desired
output voltage is achieved by varying the frequency
and amplitude of a reference or modulating voltage.
The variations in the amplitude and frequency of
the reference voltage change the pulse width
patterns of the output voltage but keep the
sinusoidal modulation. As shown in Fig 3, a lowfrequency sinusoidal modulating waveform is
compared with a high-frequency triangular
waveform, which is called the carrier waveform.
The switching state is changed when the sine
waveform intersects the triangular waveform.
The crossing positions determine the variable
switching times between states. In a single h-bridge
the switches are fired with phase shift of 1800. In a
single leg upper and lower switches are
complimentary to each other.
Fig5.Three phase output for sinusoidal pwm
Fig4 Comparison of reference wave and carrier
wave
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Fig6. FFT analysis of SPWM
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International Journal of Engineering Trends and Technology (IJETT) – Volume 11 Number 9 - May 2014
III.
THIRD HARMONIC SPWM
The sinusoidal PWM is the simplest modulation
scheme to understand but it is unable to fully utilize
the available DC bus supply voltage. Due to this
problem, the third-harmonic injection pulse-width
modulation (THIPWM) technique was developed
to improve the inverter performance. The injected
third harmonic has one sixth the amplitude of
fundamental signal.[2]. Injecting a third harmonic
component to the fundamental component gives the
following modulating waveforms for the threephase.
Van = (sin (wt) + sin (3wt)) (1)
√
Vbn =
√
(sin (wt-
) + sin (3wt)) (2)
Vbn = (sin (wt+ ) + sin (3wt))(3)
√
The THIPWM is implemented in the same manner
as the SPWM, that is, the reference waveforms are
compared with a triangular waveform. As a result,
the amplitude of the reference waveforms do not
exceed the DC supply voltage Vdc=2, but the
fundamental component is higher than the supply
voltage Vdc. As mentioned above, this is
approximately 15:5% higher in amplitude than the
normal sinusoidal PWM. Consequently, it provides
a better utilization of the DC supply voltage.
Fig9 FFT analysis of three phase voltages
IV.
CONCLUSION
In this paper the three phase three level cascaded
multilevel inverter configuration is simulated. For
this configuration sinusoidal pulse width
modulation and third harmonic sinusoidal pulse
width modulation are implemented. The inverter
performance is observed for RL load. The DC bus
utilization is increased for third harmonic spwm
when compared to sinusoidal pwm. The harmonic
distortion is observed relatively less than sinusoidal
pwm for third harmonic spwm.
REFERENCES
[1]
[2]
[3]
Fig7 Third harmonic injection
[4]
[5]
[6]
Ned Mohan, Power Electronics, Converters,
Applications and Design, third edition,2006.
J.A. Houldsworth, and D.A. Grant, “The Use of
Harmonic Distortion to Increase the Output Voltage
of a Three-Phase PWM Invertr,” IEEE Transactions
on Industry Applications, Vol. IA-20, No. 5, pp.
1124-1128, September-October 1984.
Phuong Hue Tran “Matlab/Simulink Implementation
And Analysis Of Three Pulse Width-Modulation
(Pwm) Techniques” thesis.
J. Rodriguez, J.-S. Lai, and F. Z. Peng, “Multilevel
inverters: A survey of topologies, controls, and
applications,” IEEE Trans. Ind. Electron.,vol. 49, no.
4, pp. 724–738, Aug. 2002.
J. S. Lai and F. Z. Peng, “Multilevel converter—A
new breed of power converter,” in Proc. IAS'95
Conf., 1995, pp. 2348–2356.
Surin Khomfoi and Leon M. Tolbert “Multilevel
Power Converters”
Fig8 Thre phase voltages for third harmonic spwm
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